Methacrylic acid esters of saturated aliphatic alcohols, e.g. methyl methacrylate (MMA), are useful precursors of acrylic or acrylic-type polymers. Such polymers, in turn, exhibit good transparency, weatherability and physical strength properties, and thus are eminently well suited for a wide variety of applications. Typical end uses for MMA-derived polymers include acrylic sheet that can be fabricated into signs, advertising displays, lighting fixtures, glazing materials, structural panels and the like, molding resins for automobile tail-light lenses, plumbing fixtures and the like, as well as constituents of a variety of surface coatings, adhesives, inks, floor polishes and the like.
The esterification of methacrylic acid with a saturated aliphatic alcohol such as methanol is, of course, well known in the art. One such esterification process is described in U.S. Pat. No. 3,639,460 to Wenzel et al. In that particular process the esterification reaction is carried out in what is substantially a continuous flow stirred tank reactor (CSTR) and in the presence of an excess of methacrylic acid (MA) in the reactor.
In such conventional MA-esterification processes, liquid organic by-products, typically present in appreciable concentration inasmuch as most commercial CSTRs exhibit relatively low conversion rates and provide no practical type of a purge stream, are generally undesirable. Current MA-esterification technology thus typically requires separation of such by-products from the MA, or minimization of the concentration of the by-products relative to the MA present, before the MA is esterified.
However, conventional methods for separating these liquid organic by-product substances from the crude, synthesized methacrylic acid prior to esterification are economically unattractive, principally because of unacceptable product losses. Moreover, energy costs are often significant.
The process of the present invention allows for lowcost, continuous-flow, catalytically induced, relatively high-conversion esterification of a crude methacrylic acid feed stream. The methacrylic acid-containing feed stream can contain not only the unreacted alcohol and methacrylic acid, but also appreciable amounts of by-product impurities as well. One further advantage of the present invention is that high-boiling impurities that are present can be advantageously utilized as part of the esterification medium. That is, the impurities need not be separated from the crude methacrylic acid prior to the esterification step but can be used advantageously to dilute the reactants with attendant better process control.